1. Field of the Invention
The present invention relates generally to slide fasteners and move particularly to a slide fastener stringer including a stringer tape and a series of fastener coupling elements sewn to the tape along its one longitudinal edge by utilizing a multi-thread chain stitch or "double locked stitch".
2. Prior Art
In sewing a series of fastener coupling elements to a stringer tape for a slide fastener, one of the most widely used stitch types is a multi-thread chain stitch or "double locked stitch", which is formed with two or more sewing threads, i.e. needle and looper threads. It has been customary to use spun or multifilament yarn as both of the needle and looper threads, because such non-monofilament yarns are flexible and less stretchable and hence enable the fastener elements to be sewn to the tape tightly on a high-speed sewing machine without breakage of a sewing needle.
A common problem encountered with such a prior slide fastener stringer is that, because the material and fabric structure of the modern stringer tape are usually of the type having less frictional resistance, the needle thread is liable to become loose from its cut end portions which have been cut as the fastener stringer of a continuous length has been severed into a slide fastener length. With this arrangement, when the opposite stringer tapes are laterally pulled at their one end in opposite directions during threading of a pair of the interengaged fastener stringers through a slider, the extreme one or two or even more of the fastener elements on each tape would be easily displaced. Consequently, it would be difficult or sometimes impossible to mount the slider onto the interengaged fastener stringers.
U.S. Pat. No. 3,783,476 discloses a slide fastener stringer having a row of fastener elements secured to a stringer tape by means of a single-needle double locked stitch formed with needle and looper threads, of which only the needle thread includes a monofilament yarn. The needle thread is disposed on the fastener element side of the slide fastener stringer, and therefore, the stitching must be done from that side. This requires a specially designed guide to support the slide fastener stringer such that the surface of the stringer tape on which the fastener elements are to be attached faces upward during the sewing operation. With this arrangement, a sufficient degree of tightness of the stitching is difficult to achieve.
U.S. Pat. No. 3,768,125 discloses another slide fastener stringer having a row of fastener elements secured to a stringer tape by means of a single-needle double locked stitch formed with needle and looper threads each consisting of a monofilament yarn. Not only because monofilament yarns have rigidity by nature, but also because loops of such monofilamentary looper thread extends across and over the fastener elements, a sufficient degree of flexibility of the slide fastener stringer is difficult to achieve.
FIG. 7 of the accompanying drawings illustrates, in transverse cross section, a fragment of a pair of interengaged slide fastener stringers 50,51 of the prior art in which a pair of rows of coupling elements 52,53 are attached to a pair of stringer tapes 54,55, respectively, by use of a single needle double locked stitch but with an insufficient degree of tightness. Assuming that the fastener stringer 50,51 are sharply bent in the longitudinal direction such that the top surface (undersurface in this Figure) of the tape 54,55 on which the coupling elements 52,53 are attached becomes concave, the tape edges with the sewing stitches 57,57 are displaced from the normal position toward respective connecting portions 58,58 of the opposed coupling elements 52,52, i.e. from the phantom line position to the solid line position. Therefore, the prior slide fastener stringers 50,51 would often accidentally split open when they are bent.